There has been conventionally known a lamp that is equipped with a plurality of light source units each having a mounting board on which light emitting elements are mounted, on a front surface of a substrate, wherein these plurality of light source units are disposed around the axis of the lamp with back surfaces of the respective substrates facing inside, and with gaps left from one another (refer to Japanese Patent-Laid Open No. 2013-122899, for example). In Patent Literature 1, heat radiation fins provided on the back surfaces of the substrates are formed to be parallel with the axis of the lamp.